1. Field of the Invention
The present invention relates to an apparatus and method for evaluating oil and gas reservoir characteristics. More specifically, the invention relates to tri-axial field sensors for low frequency electromagnetic fields.
2. Description of the Related Art
Various segments of the energy industry have a need for remote sensing technology to complete such tasks as evaluating oil reserves depletion, optimizing oil recovery, and monitoring underground water, gas and CO2 injection and storage.
It is well known that the ability to measure in a reliable way the electric field components in a borehole environment is of great value to implement electromagnetic-based characterization and monitoring techniques for reservoir fluids. It is also well known that permanent installation of such sensors would enable the optimization of oil field production. Current electric field sensors are very sensitive to environmental conditions such as the chemical composition of borehole fluids, the borehole temperature and pressure conditions, and fluid type, and require careful maintenance and therefore are not reliable. Without solving these problems the development of effective electromagnetic instrumentation for reservoir applications will be limited to the wireline devices based on electromagnetic induction (magnetic coils) and commercialized by major oil field service companies. These magnetic receivers based on electromagnetic induction measurements however are not optimal for measuring electric fields responses in three dimensions and at low frequency.
Current practice for looking at fluid saturation around and at a distance of several hundred meters from the borehole, relies on electromagnetic techniques using magnetometers (i.e. magnetic coils) as sensors. These devices are designed to detect and measure the induced magnetic field generated by an electromagnetic source operating in the frequency or in the time domain. This technology is typically measuring one or three components of the magnetic field.
The present state of the art therefore does not provide for three component electric field measurements to be made in a borehole environment at low frequency. U.S. Pat. No. 6,373,253, for example, discloses measuring the electromagnetic components in a borehole using a toroidal antenna in the frequency range of 1 MHz to 100 MHz. This method derives the electric field components by using an inductive method as opposed to a capacitive method and has a very high range of frequency. Where measurements are made without contact via induction, such as the method that is disclosed in U.S. Pat. No. 6,489,772, there are limitations on the sensor size and dimensions that restrict their use to single (vertical) component magnetic field, measurements.
The advantages of capacitive electric field sensors are that they do not need to directly contact the formation and that they can be made compact enough to measure all three components of the electric field even in a borehole environment. Capacitive measurements have been made in a borehole environment to measure the wellbore resistivity after injecting current into the formation as is disclosed in U.S. Pat. No. 6,809,521, but these are in a logging tool configuration and limited to a small zone in the vicinity of the borehole. The frequencies used in this technology are also high and in the range of 1 kHz to 10 kHz.
The industry is currently lacking borehole electric field sensor technology because of the difficulty of providing low contact resistance and stable conditions for the electric field sensors in the borehole. Current electric field sensors are also very sensitive to environmental conditions such as the borehole conditions and fluid type, require careful maintenance and therefore are not reliable. Otherwise, capacitive electrodes for boreholes are designed for high frequency ranges, such as greater than 1 kHz, and are used as logging tools to look in a small zone in the vicinity of the borehole. For these type of devices, the source is located with, or at most a short distance from, the receiver. The detection of the extremely small variations of the electromagnetic field that are induced by oil being replaced by injection water would benefit from the measurement of all of the three components of the electromagnetic field, which would include measuring the electric field and the magnetic field along three orthogonal directions.
Without solving these problems the development of effective remote sensing electromagnetic instrumentation for reservoir applications will be limited to the wireline devices based on electromagnetic induction (magnetic coils) and commercialized by major oil field service companies.